In the manufacturing of a luminescent screen by the conventional wet slurry process, the phosphors are deposited into openings formed in a matrix disposed on the interior surface of the faceplate, for example in the sequence: green, blue and red. This same phosphor deposition sequence is utilized in the electrophotographic screening (EPS) process described in U.S. Pat. No. 4,921,767, issued to Datta et al., on May 1, 1990. For the EPS process, a matrix having a multiplicity of openings into which the phosphors are deposited also is provided on the interior surface of the faceplate panel.
In the EPS process described in the above-referenced patent, dry-powdered, triboelectrically charged, color-emitting phosphors are deposited on a suitably prepared, electrostatically chargeable photoreceptor formed on the matrix. The photoreceptor comprises an organic photoconductive (OPC) layer overlying, preferably, an organic conductive (OC) layer, both of which are deposited, serially, on an interior surface of the CRT faceplate panel. Initially, the OPC layer of the photoreceptor is electrostatically charged to a positive potential using a suitable corona discharge apparatus. Then, selected areas of the photoreceptor are exposed to visible light to discharge those areas without substantially affecting the charge on the unexposed areas. Next, triboelectrically positively charged, green-emitting phosphor is deposited, by reversal development, onto the discharged areas of the photoreceptor to form phosphor lines of substantially uniform width and screen weight. The photoreceptor and the green-emitting phosphor are recharged by the corona discharged apparatus to impart an electrostatic charge thereon. It is desirable that the charge on the photoreceptor be of the same magnitude as that on the previously deposited green-emitting phosphor; however, it has been determined that the photoreceptor and the previously deposited phosphor do not necessarily charge to the same potential. In fact, the charge acceptance of the phosphors is different from the charge acceptance of the photoreceptor . Consequently, when different selected areas of the photoreceptor are exposed to visible light to discharge those areas to facilitate reversal development thereof with triboelectrically positively charged blue-emitting phosphor, the previously deposited green-emitting phosphor retains a positive charge of a different magnitude than the positive charge on the unexposed portion of the photoreceptor. This charge difference influences the deposition of the positively charged blue-emitting phosphor, causing it to be more strongly repelled by the charge on the previously deposited green-emitting phosphor, than by the charge on the unexposed areas of the photoreceptor. This stronger repelling effect of the green-emitting phosphor causes the blue-emitting phosphor to be slightly displaced from its desired location on the photoreceptor. The repelling effect of the prior deposited phosphor is small, nevertheless, the width of the blue-emitting phosphor lines is narrower than desired. The photoreceptor and the green- and blue-emitting phosphors are recharged by the corona discharge apparatus to impart a positive electrostatic charge thereon to facilitate the deposition of the red-emitting phosphor. The photoreceptor and the green- and blue-emitting phosphors each have a positive charge of a different magnitude thereon. Selected areas of the photoreceptor are discharged by exposure to light, while the charge on the unexposed areas of the photoreceptor and on the prior deposited phosphor is unaffected. The triboelectrically positively charged red-emitting phosphor is more strongly repelled by one of the prior deposited phosphors than by the other, in this instance the green-emitting phosphor, causing misregister of the red phosphor as it is deposited onto the discharged areas of the photoreceptor. Again, the effect is small; however, the red phosphor is slightly displaced from its desired location on the photoreceptor, resulting in a narrowing of the red phosphor lines. In addition to the effect of the prior deposited phosphors on latter deposited phosphors, the substantially uniformly charged OPC layer over the border of the matrix surrounding the useful screen area, particularly along the sides of the screen at the ends of the major axis, i.e., at the 3 o'clock and 9 o'clock positions, also exerts an effect which distorts the last phosphors lines on each side of the screen.
In order to manufacture a screen by the EPS process without the above described misregister and last line distortions, it is necessary that compensation for the repulsive effect of the matrix and the previously deposited, electrostatically-charged phosphors be provided. This invention claims a CRT having a structure that accomplishes that compensation.